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E-mail: aydinrusen@kmu.edu.tr, aydinrusen@hotmail.com Copyright by The Korean Institute of Chemical Engineers.

Optimization of gold recovery from copper anode slime by acidic ionic liquid

Aydın Rüşen† and Mehmet Ali Topçu

Department of Metallurgical and Material Engineering, Karamanoğlu Mehmetbey University, Karaman, Turkey (Received 10 April 2017 • accepted 17 July 2017)

Abstract−Hydrometallurgical gold recovery from primary or secondary sources is mainly based on a cyanide pro-cess, which is very dangerous for the environment due to the high toxicity levels. In view of the environmental effect, the present study proposes a new green solvent called 1-ethyl-3-methyl-imidazolium hydrogen sulfate (EmimHSO4)

ionic liquid (IL) to recover gold from copper anode slime (CAS). The optimum leaching conditions for maximizing gold recovery were determined by orthogonal array (OA) of Taguchi’s experimental design method. OA L16 (4

4

) includ-ing four parameters with four levels each, was used to examine the effects of IL concentration (20%, 40%, 60%, 80% v/v), temperature (25, 50, 75, 95o

C), time (½, 1, 2, 4 h) and solid/liquid ratio (1/10, 1/15, 1/20, 1/25 g/mL) on leaching effi-ciency of the gold recovery. Statistical analysis of variance (ANOVA) was used to determine the relevance between experimental conditions and gold recovery. The selective leaching tests results showed that gold recovery up to 89.07% was attained on laboratory scale under the optimum leach conditions: 80% IL concentration, 75oC, 4 h and 1/25 g/mL solid/liquid ratio. According to these results, EmimHSO4 IL provides a very good ambiance for the oxidative leaching

of gold and can be offered as an alternative leaching agent instead of harmful cyanide-based solvents. Keywords: Ionic Liquid, EmimHSO4, Anode Slime, Leach, Gold Recovery

INTRODUCTION

The world metal industry is seeking new methods to recover valuable metals in particular wastes with high efficiency and less damage to the environment [1]. Various industrial wastes are emerg-ing in every metal production process. These wastes includemerg-ing valu-able metals are called as a secondary source for several metals and gain importance increasingly. Copper anode slime (CAS) forming in step of electro-purification of anode copper is considered as the most valuable industrial waste due to the presence of valuable met-als such as Au, Ag, Se, and Te [2,3]. Many researchers have focused on the recoveries of these valuable metals from the CAS by vari-ous methods: pyrometallurgical, hydrometallurgical or combina-tion of them [4-9]. Since the nature of the slimes can vary from one refinery to another, CAS as a secondary source may have a com-plex chemical composition and physical morphology. Therefore, the applied method to recover the precious metals can be changed, depending on their chemical and morphological structures in the sources [10].

According to recent reviews [10,11] on the recovery of gold from secondary sources and conventional CAS, gold recovery from pri-mary or secondary sources is mainly based on a cyanide process, which is very dangerous for the environment due to the high tox-icity levels. In view of the environmental effects to substitute the cyanide for gold recovery, alternative leach chemicals have been sought for a long time. Common leaching agents have been exten-sively studied in the last three decades and used for the gold ex-traction from CAS: aqua-regia [12,13], thiourea [14,15],

chlorina-tion [16-18], thiocynate [19,20], thiosulfate [21], and ammonia [22]. However, they have many disadvantages, such as toxicity, corro-sion, negative effects to the environment and their evaporations at high temperature [23]. Considering the negative effects of the pres-ent solvpres-ents, the ILs called “green solvpres-ents” with less vapor pressure, better thermal stability and without harmful effect to the environ-ment have become the most promising solvents [24]. ILs as an alternative to the conventional organic solvents have a cationic and an anionic part and are used to adjust hydrophobic or hydrophilic properties [25]. However, recent studies [26,27] reported that ILs based on imidazolium are poorly bio-degradable and can stay in the aquatic environment. On the other hand, some researchers [28] revealed that the growth of micro-organisms in the environment containing imidazolium ILs is slowed down but not completely stopped, and micro-organisms continue to grow at low levels. For-tunately, recyclability and reusable of ionic liquids are very important properties to overcome environmental concerns like bio-degrada-tion. As described in previous studies [29,30], ILs based on imid-azolium cation can be recycled by different methods such as ex-traction of non-volatile products, distillation of volatile impurities etc., and reused several times after the leaching process without any change in their structures and impurities.

A limited number of studies have indicated that ILs play a sig-nificant role as a solvent or an electrolyte for recovery of precious metals in the extraction or separation processes. For the first time, 1-butyl-3-methyl-imidazolium hydrogen sulfate (BmimHSO4) IL by

mixing thiourea for gold and silver extraction from gold bearing ores was used by Whitehead et al. [30], who have also made a com-prehensive study on the usage of 1-alkyl-3-methyl-imidazolium ILs and their derivatives in order to leach sulfidic copper, gold and silver ores [31]. Dong et al. leached the chalcopyrite concentration in the temperature range of 50-90oC by using Brønsted acid (BmimHSO4)

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and its aqueous solutions [32]. Recovery of the base and precious

metals from gold ores by using BmimHSO4 and BmimCl ILs

sat-urated with potassium sulfate (K2SO4) solution in the presence of

oxidant (Peroxymonosulfate) was investigated by Whitehead et al. [33]. A recent study [34] on dissolving and recovery of gold in the presence of ILs indicated that deep eutectic solvent (DES) ILs mix-ing with choline chloride and urea/thiourea are convenient to recover gold from sulfides (galena and chalcopyrite). All these studies showed that gold can be recovered up to 85% by IL media from chalcopy-rite based ores.

Several papers have described the extraction of gold from sul-fidic ores (chalcopyrite and galena) by using ILs; nevertheless, no study related to recovery of gold from anode slime known as sec-ondary source has been reported so far. This is the first study re-ported in the literature to focus on the gold recovery from CAS by using ILs. In this study, the optimum leaching conditions for gold recovery were determined by using Taguchi experimental design method. Being simple and systematic, this method has been widely used, especially in engineering and chemistry, for product design and process optimization in last decades [35-40]. Many researchers have studied to specify the optimum parameters to recover metals such as copper [41,42], zinc [43,44] and lead [45,46]. Therefore, in the present study, optimization of the crucial parameters (concen-tration of solvents, temperature, reaction duration, solid/liquid ratio) affecting the leaching efficiency was performed to attain the maxi-mum gold recovery from CAS. In addition, ANOVA was applied to find whether leaching parameters are statistically significant. By this way, EmimHSO4 IL can be offered as a possible green leaching

agent to recover gold from CAS under the optimum condition. EXPERIMENTAL PART

1. Characterization of the Sample

CAS used in this study was provided from Er-bakır copper plant in Turkey. Before leaching experiments and characterization, the CAS was pulverized to obtain a homogeneous particle size. Fig. 1

indicates the particle size distribution of the CAS with scanning electron microscope (SEM) image, obtained by using Malvern Par-ticle Sizer (Zetasizer Nano ZS), is from 1 to 5µm. In addition, Z-Average mean known as ‘harmonic intensity averaged particle diam-eter’ of the CAS was determined as 2,807 nm.

Chemical analysis of the CAS was by fire assaying method (cupel-lation) for precious metals and inductively coupled plasma mass-spectrometry (ICP-MS) for other base metals. Chemical composi-tion of CAS in mass fraccomposi-tion is given in Table 1.

From the literature, it is known that CASs have different chemi-cal structures depending on their lochemi-cal characteristics and refinery processes. Typical CAS contains 5-41%Cu, 0.004-1.4% Au, 0.73-24% Ag, 0.2-7% Te and 0.2-3% Se [47]. However, the precious metal contents of the CAS used in this study was lower than that of the conventional ones due to the usage of scrap in the plant.

Mineralogical analysis of CAS was by an X-Ray diffractometer (XRD, Bruker D8 Advance with Da Vinci) with Cu Kα radiation at 30 kV, at a scanning rate of 0.4o min−1. According to the miner-alogical analysis, CAS was mainly composed of PbSO4, SnO2 and

Cu2O. Other minor components were detected as BaSO4, SbAsO4

and SiO2.

The sample surface’s topography and composition were revealed by SEM (Zeiss Evo LS10 model available in the Metallurgical and Materials Engineering Department of Karadeniz Technical Uni-versity) equipped with energy dispersive X-ray spectroscopy. Since elemental color mapping obtained by SEM-EDS is generally used to specify distribution of the constituents in a sample, it was applied to the original CAS. Fig. 2 shows the elemental maps with SEM photograph and EDS result for selected image area. Each color in the maps represents a separate element (Cu=red, Sn=green, Pb= dark blue, S=pink, Ba=light blue, and O=yellow).

As seen in the color mapping of the representative CAS sample given in Fig. 2, the CAS was predominantly composed of Sn, Pb, Cu, S, O and Ba elements. Additional element, Ca, was also detected by EDS analysis. The color mapping of the CAS indicated that all the elements were homogeneously scattered in the CAS, indicat-Fig. 1. Particle size distribution of the CAS with scanning electron microscope (SEM) image.

Table 1. Chemical composition of CAS in mass fraction

Element Cu Sn Pb S Ba Au (ppm) Ag (ppm)

% 23.10 20.51 15.42 4.11 5.87 21.9 2204.2

Element Ni Sb Sr Zn Bi Se (ppm) Te (ppm)

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ing high concentration of the elements in some points where cor-responding to all phases determined by XRD analysis.

2. Optimization Method

In this study, in order to determine the optimum parameters for gold recovery from CAS by Taguchi Method, experimental leach-ing conditions were selected as (a) solvent concentration, (b) tem-perature, (c) time and (d) solid/liquid ratio. These four factors with four levels for leaching of CAS are summarized in Table 2. Experi-mental parameters were investigated by using the orthogonal array (OA) experimental design method with L16 (44) given in Table 3

with experimental results.

In this study, to maximize recovery of gold from the CAS, the performance statistics of OA were evaluated by using the

follow-ing equation (Eq. (1)) [48]:

“larger is better” (1)

where (S/N)L is the performance statistic corresponding to

signal-noise ratios of “larger is better,” n is the number of repetitions for an experimental combination, and xi is the performance value of ith

experiment.

3. Leaching Experiments

ILs as an alternative to the conventional organic solvents have a cationic and an anionic part and are used to adjust hydrophobic or hydrophilic properties. According to the study by Whitehead et al. [31], when ILs based on methylimidazolium are used as a leach-ing agent to recover gold from primary ores, the amount of gold extraction decreases by increasing the length of the alkyl chain, which results in significant increase in its viscosity. Also, HSO4 is

more advantageous for gold recovery among the possible anion groups with its acidic properties, low cost and the stronger non-oxidizing anion [31]. Therefore, EmimHSO4 IL was selected as a

leaching agent based on methylimidazolium with the shortest alkyl chain as a cation group and HSO4 as an anion group. EmimHSO4

IL was commercially supplied from Merck Co. with high quality (>98%) as a new solvent to recover the gold from CAS.

After the OA experimental design, all leaching experiments were

S/N ( )L=−10log 1 N ---- 1 xi 2 ----i=1 n

⎝ ⎠ ⎜ ⎟ ⎛ ⎞ ,

Fig. 2. SEM elemental color mapping with EDS spectra for the selected area of CAS.

Table 2. Experimental parameters and their levels for leaching of CAS Parameters Levels 1 2 3 4 A IL concentration (v/v) 20 40 60 80 B Temperature (oC) 25 50 75 95 C Time (h) 0.5 1 2 4 D S/L ratio (g/mL) 1/10 1/15 1/20 1/25

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lyzed by using Perkin Elmer PinAccle 900T model atomic absorp-tion spectroscopy (AAS). Lastly, Minitab 17 Statistical Software package and ANOVA were used to analyze the collected data and to determine the effect of each parameter on the optimization criteria.

RESULTS AND DISCUSSION 1. Determination of Optimal Conditions

In the IL system, Taguchi design of experiment method was per-formed to obtain the maximum gold recovery with lowest amount of impurities. The results of gold recovery with investigated condi-conducted in closed glass flask placed on a temperature controlled

heater with a magnetic stirrer. The leaching tests were carried out at a solid-liquid ratio of 1/10 to 1/25 g/mL, using the constant vol-ume of leaching solution (50 ml) in the temperature range of 25 to 95oC. In all the experiments, the leach solution was magnetically stirred at 600 rpm. The IL concentration (v/v) of EmimHSO4 was

prepared as aqueous solutions in the range of 20% to 80% with the increment of 20% by mixing sufficiently deionized water. After each of the leaching tests, the solid and liquid parts were separated from each other by means of a filtration system equipped with a vacuum pump. Then, the amount of gold in leach liquor was ana-Table 3. L16 (4

4

) OA and experimental results of gold recovery Exp.

no.

Experimental parameters and their levels Gold recovery

(%) [S/N]L IL conc. (% v/v) Temperature (o C) Time (h) S/L ratio (g/L) 01 20 25 0.5 1/10 15.80 23.97 02 20 45 1.0 1/15 25.40 28.10 03 20 75 2.0 1/20 30.80 29.77 04 20 95 4.0 1/25 38.85 31.79 05 40 25 1.0 1/20 50.15 34.01 06 40 45 0.5 1/25 57.50 35.19 07 40 75 4.0 1/15 23.92 27.58 08 40 95 2.0 1/10 29.60 29.43 09 60 25 2.0 1/25 63.96 36.12 10 60 45 4.0 1/20 52.12 34.34 11 60 75 0.5 1/15 40.50 32.15 12 60 95 1.0 1/10 23.45 27.40 13 80 25 4.0 1/15 49.56 33.90 14 80 45 2.0 1/10 31.45 29.95 15 80 75 1.0 1/25 82.20 38.30 16 80 95 0.5 1/20 51.30 34.20 [S/N]mean 31.46

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statistic are displayed in Table 3 and Fig. 3, respectively. Selected experiments were repeated to verify the results under similar con-ditions. There is a fairly good agreement between the two experi-mental results. The experiexperi-mental average values obtained at the same conditions are displayed in Table 3.

In the Taguchi method, the graphs of marginal means for each parameter are always used to show only the trend of the each fac-tor. The numerical value of the maximum point in each graph marks the best value of that particular parameter. According to main effect plots in Fig. 3, the optimum condition for gold recovery by using EmimHSO4 can be determined as IL concentration: 80%,

Tem-perature: 75oC, Time: 4 hours and Solid/Liquid ratio: 1/25 g/mL. That is, the experimental parameter values for the optimum con-dition correspond to A4, B3, C4 and D4.

From the experimental results, the leaching of CAS by using concentrated ILs was easier when compared to leaching by its diluted aqueous solution. Because, concentrated ILs have more acidic prop-erties. Degree of acidity of the EmimHSO4 and its aqueous

solu-tions were determined by using a pH meter. At first, pH values of the original IL solutions were measured between 0.7 and 1.2 depending on the IL concentration from 20 to 80 (v/v). Increas-ing IL concentration would cause enhancement of the H+ ions in the solution as well as pH values of the IL solution, which helped to dissolve minerals or compounds in the CAS.

Several studies have mentioned that the EmimHSO4 IL is acidic

in aqueous solutions and the dissolved oxygen present in the solu-tion acts as an oxidant [31,32]. Also, it has been stated that as the ionic liquid concentration increases, the rate of dissolved oxygen in the solution increases, but the amount of dissolved oxygen decreases with increasing temperature. For this reason, in the present study, significant change in the recovery of gold was not observed as the temperature went up. It even fell at very high temperatures.

Since the presence of hydrogen sulfate anion in IL is responsible for acidity, possible reactions between acidic IL and copper

bear-ing compounds in the CAS much resembled H2SO4 as follows

(Eqs. (2)-(6)) [49]:

Cu+1/2O2+H2SO4→CuSO4+H2O (2)

Cu2O+1/2O2+2H2SO4→2CuSO4+Se+2H2O (3) Cu2Se+1/2O2+2H2SO4→2CuSO4+2H2O (4) Cu2Te+5/2O2+2H2SO4→2CuSO4+H2TeO6+2H2O (5) 2CuAgSe+O2+2H2SO4→2CuSO4+Ag2Se+2H2O (6)

2. Comparison of Predicted and Confirmation Experiment Results

After selecting the optimal level for each factor, the next step is to predict and confirm the gold recovery from CAS under this opti-mum working condition. Considering the design layout of L16 (4

4

) experimental table given in Table 3, it can be noticed that the ex-periment corresponding to the optimum condition (A4, B3, C4 and

D4) was not performed during the experimental trials. Therefore,

the value belonging to the optimum conditions should be estimated by means of the balanced characteristic of OA. In this manner, the predicted [S/N] ratio under the optimum experimental condition

of the studied parameters can be calculated theoretically by the Eq. (7) as an additive model [50-53]:

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where [S/N]mean is the arithmetical mean of the whole

experimen-tal [S/N]L ratios, [S/N]I is the arithmetical mean of [S/N]L ratios at

the optimal level for each studied parameters given in Table 4, and n is the number of the factor that affects the gold recovery from CAS. By this way, the value estimated for optimum condition could be comparable with the result of confirmation experiment.

The obtained value of [S/N]mean from Table 3 for gold recovery

is 31.46, and the values of [S/N]i for the optimum condition A4, B3,

C4 and D4 can be found from Table 4 as 34.09, 31.95, 31.90, and

35.35, respectively. The [S/N]predicted value could be obtained after

the substitution of these values into the Eq. (7); [S/N]predicted=38.38,

which can be used to estimate the gold recovery at the optimum experimental condition by substituting into Eq. (1). Thus, Eq. (1) can be rewritten as 38.38=−10log(1/(y)2

). By solving this equation, the predicted gold recovery ratio under the experimental conditions was calculated as 82.97%. On the other hand, a confirmation experi-ment was carried out at the same optimum condition to verify the predicted result. Quantities of the gold recoveries (%) confirmed experimentally and predicted theoretically with the level of opti-mum conditions for each studied parameters are summarized in Table 5.

As can be seen in Table 5, under the optimum condition the pre-dicted percentage of the gold extraction was 82.97% and the gold recovery obtained from the confirmation experiment corresponds to 89.07%. According to these results, it can be concluded that good agreement exists between the predicted and confirmed leaching efficiencies of gold.

In last three decades, several leaching agents have been used in

S N ----Predicted= S N ----mean+ S N ----i− S N ----mean ⎝ ⎠ ⎛ ⎞ i=1 n

Parameter Level 1 Level 2 Level 3 Level 4

IL concentration 28.41 31.55 32.50 34.09

Temperature 32.00 31.90 31.95 30.70

Reaction time 31.38 31.95 31.32 31.90

Solid/liquid ratio 27.23 30.89 33.08 35.35

Table 5. Confirmed experimentally and predicted theoretically recov-ery of gold quantities (%) under the optimum condition Symbol parameters Optimum experimental conditions Value Level A IL concentration (v/v) 80 4 B temperature (o C) 75 3 C time (h) 4 4 D solid/liquid ratio (g/L) 1/25 4

Confirmed recovery quantity (%) Au 89.07

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periment, maximum Au content in the pregnant solution could be obtained with minimum amount of impurities by using the pro-posed statistical method. Under the optimum condition, impuri-ties content in the filtrate were analyzed by AAS and their dissolution ratios are summarized in Table 7.

Several studies [31,32,57] have shown that ILs with acidic prop-erty have the ability to dissolve different types of minerals such as oxidic, sulfidic etc. Unfortunately, in this study, beside Au, some elements such as Cu, Sb, Ba, Se or Te in the CAS could be solved in acidic EmimHSO4 during the leaching process. Nonetheless,

other impurity elements (especially Sn and Pb) present in a large amount in the CAS could not dissolve in the IL solution due to insolubility of PbSO4 and SnO2 in the acidic media. In addition,

the low silver solubility value after the EmimHSO4 IL leaching at

optimum condition may be attributed to reaction 6.

Comparing the XRD pattern of the leach residue obtained from confirmed experiment at the optimum condition with the pattern of the original CAS sample (Fig. 4), the sulfidic and oxidic copper structures disappeared and peaks of the SnO2 and PbSO4 remained

unchanged.

3. Analysis of Variance (ANOVA) Results

The purpose of the ANOVA is to investigate which of the pro-cess parameters significantly affect the performance characteristics. gold extraction from anode slime to substitute the cyanide. Table 6

summarizes the comparison of leaching efficiencies of gold from CAS by EmimHSO4 with those by other agents.

When compared to the leaching efficiency by EmimHSO4

ob-tained under the optimum condition with those by other methods, most of them have reached relatively higher gold recovery values than that of the present study. However, with various oxidant addi-tives into EmimHSO4 ionic liquid solution, the gold recovery ratios

can be raised at high levels [33,56,57].

After filtration of the leaching mixture of the confirmation ex-Table 6. The gold leaching efficiencies from CAS by using different

leach agents

Leach agent Leach efficiency (%) Ref.

Cyanide >98% [54,55] Agua regia 80-95% [12,13] Thiourea 90-95% [14,15] Chlorination 70-99% [16-18] Thiocyanate >95% [19,20] Thiosulfate 65-88% [21] Ammonia 80-90% [22]

EmimHSO4 89% This study result

Table 7. Dissolution ratios of impurities for confirmation experiment at the optimum condition

Element Ag Cu Sn Pb Ba Ni Zn Sb Sr Bi

Dissolution rate (%) 1.12 30.22 0.12 0.07 63.07 10.26 19.14 29.47 25.93 26.63

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Statistically, the F-Test is used to specify which process parameter has a significant effect on the performance characteristic. Usually, the larger F value has the greater effect on the performance charac-teristic due to the change of the process parameter [58]. The results of the ANOVA applied in the light of such information for gold recovery from CAS are summarized in Table 8.

According to Table 8, the solid/liquid ratio and IL concentration are the most significant process parameters for gold recovery with 63.36% and 30.34% contribution rate, respectively. These results are supported from the performance statistic for Au recovery (Fig. 3) that the gold recovery sharply increased with increasing IL con-centration and solid/liquid ratio. The changes of the leaching tem-perature and duration (see Table 8) have not significant effect on the performance characteristic of gold recovery with lower than 5% contribution rates.

Ultimately, it can be stated that the gold recovery from CAS by EmimHSO4 IL is a possible hydrometallurgical technique under

the optimum condition. The maximum gold recovery with lowest amount of impurities could be yielded by Taguchi design of exper-iment method. However, obtaining the Au from this leach solution depends on the gold form in the solution. Filtrate can be processed to extract gold by several methods, such as solvent extraction with different extractants, cementation with zinc dust, ion exchange with some resins, and adsorption of Au with carbon or magnetite nano-particles [23,59]. Electrodeposition of gold from ILs is also a new applicable method proposed by several researchers [42,43,60,61]. Since Pb and Sn have not passed through the solution as contami-nants, the recovery of Au from the solution will be easier.

As described in the previous studies [29,30], ILs based on imid-azolium cation can be recycled and reused several times without any change in structure and impurity of ILs. By this way, most of the ionic liquid used in the leaching process can be sent back to the system for metal extraction without polluting the waste water. Therefore, ILs have an important role to overcome the environ-mental concern of conventional solvents. Nowadays, the bulk pro-duction costs of imidazolium-based ionic liquids are between $2.96 and $5.88 per kg [62], which is only two times of conventional sol-vents such as cyanide, thiourea and so on. As their prices fall depend-ing on the technological improvements, imidazolium-based ILs can be widely used for metal recovery as an environmentally sen-sitive solvent.

CONCLUSIONS

Taguchi method was used for determining optimum leaching

parameters (IL concentration, leaching time, temperature and solid/ liquid rate) were investigated with orthogonal experimental design, L16 (44). To maximize gold amount in the solution by using

Emi-mHSO4, IL concentration (A4): 80% (v/v), temperature (B3): 75oC,

duration (C4): 4 hours and the solid/liquid ratio (D4): 1/25 g/mL

were selected as the optimum conditions. Under this condition, gold recovery obtained from confirmation experiment (89.07%) and pre-dicted theoretical value (82.97%) indicated an excellent relation be-tween them. Statistical analysis of variance (ANOVA) revealed the relationship between experimental conditions and gold recoveries. Experimental trials on gold recovery indicated that the most effec-tive parameters were solid/liquid ratio (63.36%), IL concentration (30.34%), temperature (4.49%), and time (1.81%), respectively. Ac-cording to results, we concluded that the hydrometallurgical leach-ing process in the presence of EmimHSO4 ILs can be successfully

applied to maximize the gold recovery from CAS. ACKNOWLEDGEMENT

The authors gratefully acknowledge the Karamanoglu Mehmetbey University Scientific Research Projects (BAP) Coordinating Office for support with grant number KMU-BAP-04-M-15.

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Şekil

Table 1. Chemical composition of CAS in mass fraction
Table 2. Experimental parameters and their levels for leaching of CAS Parameters Levels 1 2 3 4 A IL  concentration  (v/v) 20 40 60 80 B Temperature  ( o C) 25 50 75 95 C Time  (h) 0.5 1 2 4 D S/L ratio (g/mL) 1/10 1/15 1/20 1/25
Table 3. L 16  (4 4 ) OA and experimental results of gold recovery Exp.
Table 5. Confirmed experimentally and predicted theoretically recov- recov-ery of gold quantities (%) under the optimum condition Symbol parameters Optimum experimentalconditions Value Level A IL concentration (v/v) 80  4 B temperature ( o C) 75  3 C time
+2

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